Bores drilled to access subterranean formations, and in particular hydrocarbon-bearing formations, are typically lined with metallic tubing, known as casing or liner. After the tubing is run into the bore, the annulus between the tubing and the surrounding bore wall is filled with cement slurry which sets to seal the annulus to prevent, for example, flow of fluid through the annulus from a high pressure formation intersected by the bore into a lower pressure formation intersected by another portion of the bore.
Casing and liner tend to be run into bores as strings of conjoined tubing sections, which strings may be up to several thousand meters long. The outer diameter of the strings will be only slightly less than the bore inner diameter and thus, particularly in extended reach and highly deviated bores, there may be considerable friction between the string and the bore tending to resist movement of the string through the bore. Also, deposits of loose material in the bore, ledges and doglegs may all serve to hinder an attempt to run a tubing string into a bore.
The end of the casing or liner string may be provided with a shoe provided with cutting or reaming elements which serve, through axial or rotational movement of the string, to dislodge, rasp or cut through bore obstructions. However, it may prove difficult to apply torque from surface to rotate such a shoe, as the connectors between adjacent sections of the string are generally not capable of withstanding any significant torque.
As noted above, once the tubing string is in place in the bore cement slurry is run down through the tubing string and into the annulus. This is achieved by pumping a slug of cement slurry of appropriate volume from surface to the leading end of the tubing, the cement slurry being isolated from other fluid in the well by appropriate leading and trailing darts or plugs. To achieve an effective cement seal between the tubing and the bore wall it is important that the fluid and any other deposits in the annulus are substantially completely displaced by the cement. This may be facilitated by rotating the string as the cement is pumped into the annulus, however as noted above it may be difficult to apply the torque necessary to rotate the string from surface, due to the frictional forces acting between the string and the bore wall.
It is among the objectives of embodiments of the invention to facilitate running in of casing and liner strings and also to facilitate cementation of such strings and thus obviate or mitigate a number of the abovementioned difficulties.